backend/remote-state/gcs: Implement an end-to-end test.

The code is loosely based on state/remote/gcs_test.go. If the
GOOGLE_PROJECT environment variable is set, this test will

1) create a new bucket; error out if the bucket already exists.
2) create a new state
3) list states and ensure that the newly created state is listed
4) ensure that an object with the expected name exists
5) rum "state/remote".TestClient()
6) delete the state

The bucket is deleted when the test exits, though this may fail if the
bucket it not empty.

backend/remote-state/gcs/backend_test.go

@@ -1,7 +1,11 @@
 package gcs
 
 import (
+	"os"
 	"testing"
+
+	"github.com/hashicorp/terraform/backend"
+	"github.com/hashicorp/terraform/state/remote"
 )
 
 func TestStateFile(t *testing.T) {
@@ -34,3 +38,89 @@ func TestStateFile(t *testing.T) {
 		}
 	}
 }
+
+func TestGCSBackend(t *testing.T) {
+	// This test creates a bucket in GCS and populates it.
+	// It may incur costs, so it will only run if the GOOGLE_PROJECT
+	// environment variable is set.
+
+	projectID := os.Getenv("GOOGLE_PROJECT")
+	if projectID == "" {
+		t.Skipf("skipping; set GOOGLE_PROJECT to activate")
+	}
+
+	const bucketName = "terraform_remote-state_test"
+	t.Logf("using bucket %q in project %q", bucketName, projectID)
+
+	config := map[string]interface{}{
+		"bucket": bucketName,
+		"prefix": "",
+	}
+
+	if creds := os.Getenv("GOOGLE_CREDENTIALS"); creds != "" {
+		config["credentials"] = creds
+		t.Logf("using credentials from %q", creds)
+	} else {
+		t.Log("using default credentials; set GOOGLE_CREDENTIALS for custom credentials")
+	}
+
+	be := backend.TestBackendConfig(t, New(), config)
+
+	gcsBE, ok := be.(*gcsBackend)
+	if !ok {
+		t.Fatalf("backend: got %T, want *gcsBackend", be)
+	}
+
+	ctx := gcsBE.storageContext
+
+	// create a new bucket and error out if we can't, e.g. because it already exists.
+	if err := gcsBE.storageClient.Bucket(bucketName).Create(ctx, projectID, nil); err != nil {
+		t.Fatalf("creating bucket failed: %v", err)
+	}
+	t.Log("bucket has been created")
+
+	defer func() {
+		if err := gcsBE.storageClient.Bucket(bucketName).Delete(ctx); err != nil {
+			t.Errorf("deleting bucket failed: %v", err)
+		} else {
+			t.Log("bucket has been deleted")
+		}
+	}()
+
+	// this should create a new state file
+	_, err := be.State("TestGCSBackend")
+	if err != nil {
+		t.Fatalf("State(\"TestGCSBackend\"): %v", err)
+	}
+
+	states, err := be.States()
+	if err != nil {
+		t.Fatalf("States(): %v", err)
+	}
+
+	found := false
+	for _, st := range states {
+		if st == "TestGCSBackend" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("be.States() = %#v, missing \"TestGCSBackend\"", states)
+	}
+
+	// ensure state file exists
+	if _, err := gcsBE.storageClient.Bucket(bucketName).Object("TestGCSBackend.tfstate").Attrs(ctx); err != nil {
+		t.Fatalf("Attrs(\"TestGCSBackend.tfstate\"): %v", err)
+	}
+
+	c, err := gcsBE.client("TestGCSBackend_remote_TestClient")
+	if err != nil {
+		t.Fatal(err)
+	}
+	remote.TestClient(t, c)
+
+	if err := be.DeleteState("TestGCSBackend"); err != nil {
+		t.Errorf("DeleteState(\"TestGCSBackend\"): %v", err)
+	}
+}